There is a steady increase globally in the installation of reverse osmosis (RO) desalination plants. Simultaneously there is a growing concern about energy for desalination. Membrane fouling is potentially a major inefficiency in a desalination (and reclamation) plant and the reduced permeability produces an unwanted increase in energy demand as well as costly shut down and membrane replacement.
Biofouling is widely recognized as the major fouling issue in both RO desalination and reclamation plants. Current pre-treatment options could not eliminate biofilm formation in the membrane modules, each of which consists of multiple membrane layers and, feed and permeate channel spacers. As a result, productivity is reduced or more energy is required to maintain the production, and the salt passage through the membrane modules is aggravated. The lack of suitable techniques sensitive enough for studying and/or detecting membrane biofouling under realistic operating conditions has hindered the development of appropriate strategies to control biofouling, which is critical for the sustainability of RO membrane technology.
Conventional techniques such as monitoring the transmembrane pressure profile under constant flux operation or the flux profile under constant pressure operation, quality of permeate and microbial monitoring suffer from one or more disadvantages such as low sensitivity or long response time to detect incipient fouling. U.S. Pat. No. 6,161,435 (Bond et al.) describes an acoustic detection technique for the initiation of membrane fouling as well as the state of membrane fouling and the rate of membrane fouling to provide an early warning that permits adjusting the system operating parameters to mitigate the fouling problem. It uses the propagation of acoustic waves via compression and rarefaction to give information on the physical characteristics of the media through which the waves travel. However, it has very limited applicability for detecting biological or organic fouling in membrane processes. In particular, Bond et al. is not able to detect the initiation of membrane biofouling, the state of biofouling or the rate of biofouling caused by biological and/or organic foulants.
Furthermore, employing the apparatus of Bond et al. on commercial spiral wound modules is very costly given the huge number of transducers required to cover an appropriate effective membrane area for each spiral wound module multiplied by the significant number of modules used in a typical desalination or reclamation plant. This is further aggravated by the sheer complexity of the resultant acoustic signature from multiple reflections of the membranes, spacers and supports, resulting in lower sensitivity due to the increased superimposition of transmitted and reflected waveforms. It is also required for the transducer to be custom-made made due to the surface configuration of the spiral wound module.